'Honest conversation'

Ms Caldwell, from Castlederg, said she was "absolutely devastated" to have the supply confiscated after she declared it to border officials.

Ms Caldwell later met Mr Hurd at the Home Office to plead with him "parent to parent" to get the oil back.

"It's Billy's anti-epileptic medication that Nick Hurd has taken away, it's not some sort of joint full of recreational cannabis," she said.

"We had an honest and genuine conversation. I have asked him to give Billy back his medicines, but he said no."

She also warned of the dangers of Billy missing his first dose of cannabis oil in 19 months.

"The reason they don't do it is that it can cause really bad side-effects – they wean them down slowly," she said.

"So what Nick Hurd has just done is most likely signed my son's death warrant."

'Back to Canada'

A Home Office spokeswoman said it was "sympathetic to the rare situation that Billy and his family are faced with".

"The policing minister met Ms Caldwell and advised her that despite these extremely difficult circumstances, it is unlawful to possess Schedule 1 drugs such as those seized at the border this morning without a licence," she added.

"The minister urged the family to explore licensing options with the Department of Health Northern Ireland."

The science

CBD and Tetrahydrocannabinol (THC) are two types of cannabinoids found naturally in the resin of the marijuana plant.

Unlike THC, pure CBD oil is not a psycho-active ingredient associated with the "high" in marijuana.

However, the Medicines and Healthcare products Regulatory Agency (MHRA) is of the opinion that CBD products, used for a medical purpose, are medicines.

CBD oil has not yet been licensed in the UK as a medicine but can be prescribed by doctors in special circumstances.

The oil containing the THC chemical is illegal under the misuse of drugs legislation.